1. Field of the Invention
The present invention relates to a parallel transmission system, and in particular, to an optical parallel transmission system using a plurality of optical fibers, for communicating between boards or between apparatuses in communication within an information processing system or within an information transmission switching system.
2. Description of the Related Art
Recently, as the speed of information processing becomes higher, the amount of information being processed has increased remarkably. This is mainly due to improvements in the ability of information processors, and therefore, it is also required to improve the speed of communication between boards or between apparatuses.
Communication between boards or apparatuses has so far been performed using metal cables transmitting electrical signals, such as coaxial cables and pair cables. In the coaxial cables or the pair cables, however, an amount of transmitted information, a transmission speed, and a transmission distance tend to be limited by physical factors. Namely, in order to rapidly and correctly transmit a larger amount of information, the cable is required to be thicker. Thus, physical limitations in thickness and weight of the cables arise, and transmission distances are limited because of deterioration of information occurring if the transmission distance is increased.
In addition, flexibility has been required in arrangement of terminal equipment and peripheral equipment within an office, to create a better working environment which has been increasingly desired. However, coaxial cables do not completely satisfy the aforementioned requirement because of the physical limitations and the limitation in the transmission distance.
Furthermore, in broadband aspects of integrated services digital networks (B-ISDN) which will attract much attention as a communication technology in the next generation, a transmission switching system handles not only voice data for telephone sets but also data for pictorial communications and data for facsimile communications. Since these data are multiplexed and transmitted all at once, the system must transmit thousands of times as much data as in the case where only voice data are transmitted. Therefore, metal cables such as the coaxial cables and the pair cables cannot adequately deal with so much data.
An optical parallel transmission technology that is capable of transmitting a large amount of information and that negligibly deteriorates information even over a long transmission distance, has attracted much attention. For example, a 4B6B coding as a coding technology for optical parallel transmissions has been published in "N. Yoshida et al.; A Study on Optical Parallel Transmission Systems; National Convention Record, 1987, IEICE, 2408".
In a system described in the above publication, each serial data constituting continuously transmitted parallel data are partitioned into data segments of four bits, and each of the data segments are coded into balanced code of six bits and converted into optical signals, at a transmitter side. The optical data signals are transmitted with a clock signal. In a receiver side, received optical signals are converted into electrical signals and the six bit codes are sequentially decoded into four bit data.
In the aforementioned system, complex circuits such as a 4B6B coder, recorder, a memory, and a clock rate changing circuit, are required in the transmitter and the receiver sides. Namely, investigations into simplification and lowering of power consumption have not appeared to be sufficient regarding an optical parallel link. The simplification and lowering of power consumption are important subjects in optical parallel transmission systems.